The patent and technical literature disclose that there have been many attempts to incorporate lightweight particles into a cementitious mix to provide lightness without sacrificing an undue amount of strength, and to produce a satisfactory commercial structural product. The obvious advantages of lightweight structural units are to reduce freight and shipping costs and to facilitate handling of the structural units. At the same time, such units must be of sufficient strength to withstand a certain amount of rough handling during the manufacturing process, as well as in loading, unloading and storage of final product, and while they are being assembled into their structural use. Cementitious materials and especially calcined gypsum when cured in relatively thin strata or sheets have a decided brittleness.
Lighter weight structural units can be made in larger sizes, if desired, which can also reduce manufacturing costs per unit of area.
Experience has taught that entrainment of air alone will not product a sufficiently light wallboard that has adequate strength. Further, the incorporation of lightweight particles, alone, even at a low density, does not produce a satisfactory lightweight product.
Problems that have been encountered, lie in the area of dispersing the lightweight articles thoroughly throughout the wet cement or calcined gypsum mixture, and firmly adhering such particles in the cured cementitious matrix. Originally, technologies for decreasing the weight of structural units involved the use of expanded vermiculite, perlite and the like. In more recent times, the lightweight particles used have been particles or beads of expandable thermoplastic polymeric resins, primarily expandable polystyrene beads, as well as polyethylene and various polymeric copolymers, which are generally expanded before use to achieve the lowest density. These thermoplastic particles or beads are advantageous in that each particle or bead comprises closed cells and when they are pre-expanded may be readily made to as low a density as 1 pound per cubic foot or even as low as about 0.5 or 0.6 pounds/cubic foot. Generally, preexpansion on a commercial basis will provide particles from as low as about 0.6 pounds per cubic foot to about 11/2 pounds per cubic foot without extra and costly equipment and additional processing. Thus, the thermoplastic lightweight particles are a great deal more advantageous than those priorly used of vermiculite and/or perlite which have a density at least several times that of the thermoplastic granules.
Brittleness of the cementitious material also results in a less desirable product, especially when lightweight products containing low density thermoplastic granules or particles. In a lightweight wallboard, the volume occupied by the thermoplastic and/or entrained air materially reduces the amount of gypsum matrix present and lowers strength and especially the desired flexural strength.
Another major problem that has been encountered in the high speed commercial production of lightweight structural units has been the removal of excess water from the slurry or mixture during the curing. A certain amount of water is required for the water or hydration to react with the dry cementitious material such as calcined gypsum or cement. To obtain a free flowing mixture it has been necessary to add excess water on the order of two or three or more times that actually needed for hydration to provide a smooth, free-flowing, low viscosity mixture suitable for use in transporting and placing the mix into a mold or other means where the slurry or mix is dried to form the core for the lightweight structure. The energy, time, equipment and space required for the removal of the excess moisture create a critical obstacle in the low cost manufacture of lightweight, high speed, high volume production of structural units, such as gypsum wallboard and the like.